Impedance analysis of automotive high voltage networks for EMC measurements

The increasing demand of electric power in electric vehicles cannot be supplied by conventional automotive power networks. Therefore novel power bus systems, capable to deliver the required power are under development. These bus systems have higher operating voltages, which result in lower current amplitudes for power transmission. As power efficiency is one of the main factors determining the range of an electric car with a given battery capacity, fast slew rates of the power semi-conductors are preferred. But with increasing slew rates also emitted radio frequency (RF) disturbances increase. To reduce electromagnetic interferences (EMI) of power electronics a complete shielding of the power bus system is necessary. Thus the network configuration is changed from unshielded single core multiwire harnesses into coaxial conductor layouts, and therefore also the impedance characteristics of the entire network are changed. Electromagnetic compatibility (EMC) of automotive components is measured according to CISPR 25, substituting the vehicle environment with line impedance stabilization networks (LISN). Recent research shows that LISNs, developed for low voltage networks, are not ideal to measure conducted emissions of high voltage (HV) components because of changed characteristic impedances and additional shielding. This paper deals with a method of determining the high-frequency impedances of automotive HV power networks. A Vector Network Analyzer (VNA) is used to measure Scattering parameters of different HV power cables and an automotive Li-Ion accumulator battery. Matrix conversions allow calculating an impedance network, which is able to represent an automotive HV networks.